Add tests for the long double version of ecvt and fcvt

The test case misc/tst-efgcvt.c only tests the double variants of the
Old-fashioned System V number-to-string functions: ecvt, fcvt, and their
re-entrant counterparts.  With a few macros, the code can be reused for
the long double versions of these functions.  A future patch will reuse
it again for IEEE long double on powerpc64le.

Tested for powerpc and powerpc64le.