pkg/55818: firefox build on 32-bit x86 is possible without libatomic

[Onno van der Linden <o.vd.linden%quicknet.nl@localhost>]

>Number:         55818
>Category:       pkg
>Synopsis:       firefox build on 32-bit x86 is possible without libatomic
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Nov 22 22:25:00 +0000 2020
>Originator:     Onno van der Linden
>Release:        NetBSD 9.99.75
>Organization:
>Environment:
System: NetBSD sheep 9.99.75 NetBSD 9.99.75 (SHEEPKMS) #0: Sat Nov 14 12:22:46 CET 2020 onno@sheep:/usr/src/sys/arch/i386/compile/SHEEPKMS i386
Architecture: i386
Machine: i386
>Description:
pkg/55456 describes failing to compile firefox on NetBSD/i386 9.0
due to __atomic_load unresolved external. 

I've build firefox 74 on NetBSD/i386 9.99.52 with
clang 9.0 and rust 1.42 without the need for libatomic
I've also build firefox 83 on NetBSD/i386 9.99.75 with
clang 10.0.1 and rust 1.45 without the need for libatomic
>How-To-Repeat:
Build firefox on NetBSD/i386 using the packages described earlier.
>Fix:
-march=i586 has worked for a long, long time to get the __atomic_*
functions without using libatomic for gcc
I'm 100% certain that clang since version 9.x works the same way
as gcc has done for years. No idea what clang version was used
in pkg/55456.

Looks like the libatomic inclusion in mozilla-common.mk
for NetBSD/i386 could (should ?) be me made dependant on
the clang version used to compile firefox.