[rs6000] Rewrite the declaration of a variable

[rs6000] Rewrite the declaration of a variable

Rewrite the declaration of toc_section from the source file rs6000.c to its
header file for standardizing the code.

Bootstrap and regression were done on powerpc64le-linux-gnu (LE) with no
regressions.

gcc/ChangeLog

2020-03-20  Bin Bin Lv  <shlb.linux.ibm.com>

	* config/rs6000/rs6000-internal.h (toc_section): Remove the
	declaration.
	* config/rs6000/rs6000.h (toc_section): Add the declaration.
	* config/rs6000/rs6000.c (toc_section): Remove the declaration.