Fix write endianness/size problem for fast tracepoint enabled flag

I am sending this fix on behalf of Par Olsson, as a follow-up of this
one:

https://www.sourceware.org/ml/gdb-patches/2015-10/msg00196.html

This problem is exposed when enabling/disabling fast tracepoints on big
endian machines.  The flag is defined as an int8_t, but is written from
gdbserver as an integer (usually 32 bits).  When the agent code reads it
as an int8_t, it only considers the most significant byte, which is
always 0.

Also, we were writing 32 bits in an 8 bits field, so the write would
overflow, but since the following bytes are padding (the next field is
an uint64_t), it luckily didn't cause any issue on little endian
systems.

The fix was originally tested on ARM big endian systems, but I don't
have access to such a system.  However, thanks to Marcin's PowerPC fast
tracepoint patches and gcc110 (big endian Power7) on the gcc compile
farm, I was able to reproduce the problem, test the fix and write a
test (the following patch).

gdb/gdbserver/ChangeLog:

YYYY-MM-DD  Par Olsson  <par.olsson@windriver.com>

	* tracepoint.c (write_inferior_int8): New function.
	(cmd_qtenable_disable): Write enable flag using
	write_inferior_int8.