[gdb/testsuite] Prevent timeout in gdb.ada/float-bits.exp

Recent commit 32a5aa26 ("[gdb/testsuite] Fix gdb.ada/float-bits.exp
for powerpc64le") started using command "maint print architecture", which
produces ~275 lines.

Rewrite the corresponding gdb_test_multiple to read line-by-line, to prevent
timeouts on slower test setups.

Note that this doesn't fix a timeout in the test-case on aarch64 due to:
...
gdbarch_dump: read_core_file_mappings = <0x817438>
(gdb) aarch64_dump_tdep: Lowest pc = 0x0x8000
...

Tested on x86_64-linux.