libphobos: Merge upstream phobos fb4f6a713

Improves the versioning of IeeeFlags and FloatingPointControl code and
unit-tests, making it clearer which targets can and cannot support it.

Reviewed-on: https://github.com/dlang/phobos/pull/7435

libphobos/libdruntime/MERGE

-6c45dd3a6523a21887cb9a883eeb3abd40375dc1
+c9c209e2c62ce43a2c08ddd61d647730716b2d0f
 
 The first line of this file holds the git revision number of the last
 merge done from the dlang/druntime repository.

libphobos/src/MERGE

-68cc18adbcdbf2a62cb85a5cb2a34236af2ab05a
+fb4f6a713f5b78742f93e072cff6a6c4ecf9323d
 
 The first line of this file holds the git revision number of the last
 merge done from the dlang/phobos repository.