Use +secure-plt for powerpc-unknown-linux-gnu{,spe}

[taiki-e]

Fixes #136131
See that issue for details.

I'm not sure about the policy about baseline on these platforms (there is no platform support doc for them), but it seems that the Debian/Ubuntu's cross-compiler (powerpc-linux-gnu-gcc) already uses --enable-secureplt at least as of Debian 9 (stretch) and Ubuntu 14.04.

$ cat /etc/os-release | grep VERSION_ID
VERSION_ID="9"
$ powerpc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=powerpc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/powerpc-linux-gnu/6/lto-wrapper
Target: powerpc-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 6.3.0-18' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-powerpc-cross/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-powerpc-cross --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-powerpc-cross --with-arch-directory=ppc --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libgcj --enable-objc-gc=auto --enable-secureplt --disable-softfloat --with-cpu=default32 --disable-softfloat --enable-targets=powerpc-linux,powerpc64-linux --enable-multiarch --with-long-double-128 --enable-multilib --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=powerpc-linux-gnu --program-prefix=powerpc-linux-gnu- --includedir=/usr/powerpc-linux-gnu/include
Thread model: posix
gcc version 6.3.0 20170516 (Debian 6.3.0-18) 

$ cat /etc/os-release | grep VERSION_ID
VERSION_ID="14.04"
$ cat /etc/debian_version 
jessie/sid
$ powerpc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=powerpc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/powerpc-linux-gnu/4.8/lto-wrapper
Target: powerpc-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.4-2ubuntu1~14.04.1' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/powerpc-linux-gnu/include/c++/4.8.4 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --disable-libitm --disable-libsanitizer --disable-libquadmath --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-powerpc-cross/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-powerpc-cross --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-powerpc-cross --with-arch-directory=ppc --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libgcj --enable-objc-gc --enable-secureplt --disable-softfloat --with-cpu=default32 --disable-softfloat --enable-targets=powerpc-linux,powerpc64-linux --enable-multiarch --disable-werror --with-long-double-128 --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=powerpc-linux-gnu --program-prefix=powerpc-linux-gnu- --includedir=/usr/powerpc-linux-gnu/include
Thread model: posix
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04.1) 

cc @glaubitz (who added powerpc-unknown-linux-gnuspe in #48484)

r? tgross35

@rustbot label +O-PowerPC +O-linux-gnu

try-job: dist-powerpc-linux
try-job: dist-powerpc64-linux
try-job: dist-powerpc64le-linux
try-job: dist-various-1
try-job: dist-various-2
try-job: aarch64-gnu