[PATCH] Run gdb.compile/*.exp on {x86,x86_64,s390}-linux only

[Pedro Alves]

> searching for compiler matching regex ^aarch64(-[^-]*)?-linux(-gnu)?-gcc$^M
> found compiler /home/yaoqi01/fsf-trunk-build/build-native-aarch64-none-linux-gnu/install/sysroot/usr/bin/aarch64-none-linux-gnu-gcc^M
> Passing 11 compiler options:^M
> Compiler option 0: <>^M
> Compiler option 1: <-std=gnu11>^M
> Compiler option 2: <-fno-exceptions>^M
> Compiler option 3: <-O0>^M
> Compiler option 4: <-gdwarf-4>^M
> Compiler option 5: <-fPIE>^M
> Compiler option 6: <-Wall>^M
> Compiler option 7: <-Wno-implicit-function-declaration>^M
> Compiler option 8: <-Wno-unused-but-set-variable>^M
> Compiler option 9: <-Wno-unused-variable>^M
> Compiler option 10: <-fno-stack-protector>^M
> source file produced: /tmp/gdbobj-6vzbkt/out3.c^M
> ^M
> aarch64-none-linux-gnu-gcc: error: : No such file or directory

It sounds like the empty argument passed as "Compiler option 0" is
being interpreted by the compiler as a source file name?  And then
the error message is including the file name that is not found, 
but since the file name is the empty string, it reads a bit
cryptic.

The fix is probably to not pass that option at all if empty.

Thanks,
Pedro Alves