[gdb/testsuite] Fix typo in gdb.multi/multi-arch-exec.exp

[gdb/testsuite] Fix typo in gdb.multi/multi-arch-exec.exp

With gdb.multi/multi-arch-exec.exp I run into:
...
Running src/gdb/testsuite/gdb.multi/multi-arch-exec.exp ...
ERROR: tcl error sourcing src/gdb/testsuite/gdb.multi/multi-arch-exec.exp.
ERROR: wrong # args: extra words after "else" clause in "if" command
    while executing
"if [istarget "powerpc64*-*-*"] {
        set march "-m64"
    } else if [istarget "s390*-*-*"] {
        set march "-m31"
    } else {
        set march "-m32"
    }"
...

Fix the else if -> elseif typo.

Tested on x86_64-linux.