btrace, testsuite: fix assembly source file selection

btrace, testsuite: fix assembly source file selection

Some btrace tests use assembly source files.  They use the target triplet to
distinguish between x86_64 and ia32 ISA.  This does not work for -m32 tests
without setting the target triplet to i686-?-?.

Instead use is_amd64_regs_target to distinguish between x86_64 and ia32 ISA.

See also https://sourceware.org/ml/gdb-patches/2016-07/msg00256.html.

testsuite/
	* gdb.btrace/record_goto.exp: Use is_amd64_regs_target for selecting
	assembly source files.
	* gdb.btrace/stepi.exp: Use is_amd64_regs_target for selecting
	assembly source files.
	* gdb.btrace/tailcall.exp: Use is_amd64_regs_target for selecting
	assembly source files.
	* gdb.btrace/tailcall-only.exp: Use is_amd64_regs_target for selecting
	assembly source files.