- Switch to clang_build globally; just on TW/x86_64 it does not work
due to unreolved externals `__rust_probestack' - disable clang_build
then.
- useccache: Add conditionals to enable/disable ccache.

- Mozilla Firefox 87.0
* requires NSS 3.62
* removed obsolete BigEndian ICU build workaround
* rebased patches
MFSA 2021-10 (bsc#1183942)
* CVE-2021-23981 (bmo#1692832)
Texture upload into an unbound backing buffer resulted in an
out-of-bound read
* CVE-2021-23982 (bmo#1677046)
Internal network hosts could have been probed by a malicious
webpage
* CVE-2021-23983 (bmo#1692684)
Transitions for invalid ::marker properties resulted in memory
corruption
* CVE-2021-23984 (bmo#1693664)
Malicious extensions could have spoofed popup information
* CVE-2021-23985 (bmo#1659129)
Devtools remote debugging feature could have been enabled
without indication to the user
* CVE-2021-23986 (bmo#1692623)
A malicious extension could have performed credential-less
same origin policy violations
* CVE-2021-23987 (bmo#1513519, bmo#1683439, bmo#1690169,
bmo#1690718)
Memory safety bugs fixed in Firefox 87 and Firefox ESR 78.9