Fix hex floating point lexing

PR gdb/18318 notes that gdb will sometimes incorrectly handle hex
floating point input.  This turns out to be a bug in the C lexer; the
'p' was not being correctly recognized, and so the exponent was not
always passed to the floating point "from_string" method.

Tested by the buildbot "Fedora-x86_64-m64" builder.

gdb/ChangeLog
2020-06-11  Tom Tromey  <tom@tromey.com>

	PR gdb/18318:
	* c-exp.y (lex_one_token): Handle 'p' like 'e'.

gdb/testsuite/ChangeLog
2020-06-11  Tom Tromey  <tom@tromey.com>

	PR gdb/18318:
	* gdb.base/printcmds.exp (test_float_accepted): Add more hex
	floating point tests.