Hmm ... My last line looks the same like Rich's, but different to yours:

# cat /sys/devices/system/cpu/vulnerabilities/spectre_v2
Mitigation: Full AMD retpoline, STIBP: disabled, RSB filling

I don't have IBPB mentioned in there at all.  I'm on gentoo-sources-4.19.57. 
Are you running a later kernel?

According to this article a microcode update seems to be necessary, but I'm
not sure if this statement only applies to Intel CPUs:

https://access.redhat.com/articles/3311301#indirect-branch-prediction-barriers-ibpb-10


My piledriver output from an old 4.19 has IBPB, so given that redhat info, it looks like you do have old microcode. I don't pass anything via the kernel command line, as I assume the defaults are good.

$ cat kern-4.19.7-vuln.txt
/sys/devices/system/cpu/vulnerabilities/l1tf:Not affected
/sys/devices/system/cpu/vulnerabilities/meltdown:Not affected
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Mitigation: Speculative Store Bypass disabled via prctl and seccomp
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: __user pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Full AMD retpoline, IBPB: conditional, STIBP: disabled, RSB filling

FWIW
$ md5sum /lib/firmware/amd-ucode/microcode_amd_fam15h.bin
3bdedb4466186a79c469f62120f6d7bb  /lib/firmware/amd-ucode/microcode_amd_fam15h.bin