gas: support .half, .word and .dword directives in eBPF

This little patch adds support to the eBPF port of GAS for a few data
directives.  The names for the directives have been chosen to be
coherent with the suffixes used in eBPF instructions: b, h, w and dw
for 8, 16, 32 and 64-bit values respectively.

Documentation and tests included.
Tested in a x86_64 host.

gas/ChangeLog:

2019-07-17  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* config/tc-bpf.c (md_pseudo_table): .half, .word and .dword.
	* testsuite/gas/bpf/data.s: New file.
	* testsuite/gas/bpf/data.d: Likewise.
	* testsuite/gas/bpf/data-be.d: Likewise.
	* testsuite/gas/bpf/bpf.exp: Run data and data-be.
	* doc/c-bpf.texi (BPF Directives): New section.