firmware: Map '@' in filenames to '_' in symbols.

'@' is not a valid character in symbol names and can sometimes appear
in path names.

Reviewed by: imp, markj
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D35480

(cherry picked from commit 628a4156a761d3daac8a4adc0e675c6e6064fd5a)