Avoid TestNullDlsym hanging on HP-PA (#141011)

glibc has no support for IFUNC on HP PA RISC yet. Rather than waiting
for an "OK" that we'll never get, skip the test.

See: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/hppa/dl-irel.h;h=770dcb3ea3a6ce232bb11bf59315cfec4c543f93;hb=HEAD