firefox: a couple patches to fix build on arm

1) disable elf hack, as it is not compatible with arm
2) disable hardcoded optimization and debug flags,
   as debug symbols heavily increase memory usage
3) disable usage of optldflags on arm, as they may contain
   -s -Wl,--gc-sections which breaks the linker (issue #186)

i've yet to build firefox successfully on arm, as linking it with
ld.bfd uses way more than 1.5 GB RAM:
https://bugzilla.mozilla.org/show_bug.cgi?id=984515
and ld.gold is too buggy:
https://sourceware.org/bugzilla/show_bug.cgi?id=16728

and each run takes 2.5 days, so it's a PITA to debug/fix.