[gdb/testsuite] Remove target limits in gdb.base/catch-syscall.exp

In test-case gdb.base/catch-syscall.exp, proc test_catch_syscall_multi_arch we
test for supported targets using istarget, like so:
...
    if { [istarget "i*86-*-*"] || [istarget "x86_64-*-*"] } {
        ...
    } elseif { [istarget "powerpc-*-linux*"] \
                  || [istarget "powerpc64*-linux*"] } {
        ...
...
but the tests excercised there can all be executed if gdb is configured with
--enable-targets=all.

Rewrite the proc to iterate over all cases, and check if the test is supported
by trying "set arch $arch1" and "set arch $arch2".

Tested on x86_64-linux, with:
- a gdb build with --enable-targets=all, and
- a gdb build build with my usual --enable-targets setting (too long to
  include here) which means the sparc vs sparc:v9 case is unsupported.