Allow the .cfi_sections directive to be reissued provided that CFI generation has...

Allow the .cfi_sections directive to be reissued provided that CFI generation has not yet started.

	PR gas/19614
	* dw2gencfi.c (cfi_sections_set): Delay setting this variable
	until it is actually used.
	(cfi_set_sections): Set cfi_sections_set to true.
	(dot_cfi_startproc): Likewise.
	(dot_cfi_endproc): Likewise.
	(dot_cfi_fde_data): Likewise.
	(cfi_finish): Likewise.
	(dot_cfi_sections): Do not set cfi_sections_set.
	* doc/as.texinfo (.cfi_sections): Note that targets can provide
	their own cfi section name.  Also note that the directive can be
	reissued provided that CFI generation has not started.
	* testsuite/gas/mips/compact-eh-err2.s: Add .cfi_startproc and
	.cfi_endproc directives so that the redefinition of .cfi_sections
	will trigger the generation of the error message.
	* testsuite/gas/mips/compact-eh-err2.l: Update expected line
	number of error message.