i386: Generate lfence with load/indirect branch/ret [CVE-2020-0551]

Add 3 command-line options to generate lfence for load, indirect near
branch and ret to help mitigate:

https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00334.html
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-0551

1. -mlfence-after-load=[no|yes]:
  -mlfence-after-load=yes generates lfence after load instructions.
2. -mlfence-before-indirect-branch=[none|all|memory|register]:
  a. -mlfence-before-indirect-branch=all generates lfence before indirect
  near branches via register and a warning before indirect near branches
  via memory.
  b. -mlfence-before-indirect-branch=memory issue a warning before
  indirect near branches via memory.
  c. -mlfence-before-indirect-branch=register generates lfence before
  indirect near branches via register.
Note that lfence won't be generated before indirect near branches via
register with -mlfence-after-load=yes since lfence will be generated
after loading branch target register.
3. -mlfence-before-ret=[none|or|not]
  a. -mlfence-before-ret=or generates or with lfence before ret.
  b. -mlfence-before-ret=not generates not with lfence before ret.

A warning will be issued and lfence won't be generated before indirect
near branch and ret if the previous item is a prefix or a constant
directive, which may be used to hardcode an instruction, since there
is no clear instruction boundary.

	* config/tc-i386.c (lfence_after_load): New.
	(lfence_before_indirect_branch_kind): New.
	(lfence_before_indirect_branch): New.
	(lfence_before_ret_kind): New.
	(lfence_before_ret): New.
	(last_insn): New.
	(load_insn_p): New.
	(insert_lfence_after): New.
	(insert_lfence_before): New.
	(md_assemble): Call insert_lfence_before and insert_lfence_after.
	Set last_insn.
	(OPTION_MLFENCE_AFTER_LOAD): New.
	(OPTION_MLFENCE_BEFORE_INDIRECT_BRANCH): New.
	(OPTION_MLFENCE_BEFORE_RET): New.
	(md_longopts): Add -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(md_parse_option): Handle -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(md_show_usage): Display -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.
	(i386_cons_align): New.
	* config/tc-i386.h (i386_cons_align): New.
	(md_cons_align): New.
	* doc/c-i386.texi: Document -mlfence-after-load=,
	-mlfence-before-indirect-branch= and -mlfence-before-ret=.